The first part of Mold remediating is stopping the moisture issue  until you figure out that part don't bother with step 2 . 

Gently let them know that if they continue to use the HVAC in this manner and it causes the unit to fail, they will be responsible for replacing it due to negligence. 

It is fine to take a gentle approach when talking to them, but those provisions are in your lease for a reason.

Bringing the $750 Tenant to the Real WorldAs for your tenant enjoying their $750-a-month sweet deal, it's time for a gentle reality check.

They also make those moisture meters, that I have had roofers use in my attics to attempt to trace back sources.As noted, in my market, if you have active leaks that you are aware of, they need to be disclosed when selling a property.  

Mold requires moisture and a food source (dust is sufficient).

And they are being very kind and gentle with their thoughts etc..

They pull any moisture they can get out of the air, so if there are any small cracks or openings to outside air they will just continue running, filling up their tank from the moisture in the outside air and not working efficiently at drying out the space you’re trying to dry very well.

Super rookie here so be gentle...I am exploring the option for a possible rent-to-own situation.

@Tony Sherman, yes.Closing delays on buy side with title issuesMaterial delays, which can compound into contractor delays when you try to keep schedules tightContractor delays because prior jobs are not wrapping on timeContractor delays because you need to fire one and hire a new oneScope of work issues, i.e. no signs of moisture intrusion prior to buying, but all of a sudden you have a wet floor after a good hard rain.Closing issues on sale sideHow do you manage?